The present invention concerns the domain of the paper industry and more specifically the preparation of pulp from recycling used papers to produce paper.
For the manufacture of pulp from used paper, it is necessary to put the cellulose fibres in suspension and discard the undesirable foreign components, called contaminants, being this operation called decontamination. The contaminants can have miscellaneous shapes. In particular, metallic particles (clips), sand and gravel, adhesive materials, bits of plastic, . . . are found.
Inks are also found, which will be important to remove for some productions, namely for papers called  less than  less than white greater than  greater than  aimed to be printed, to be handwritten or for sanitary use ( less than  less than tissue greater than  greater than ).
In addition to the above-mentioned contaminants, mineral matter is also found, blended to the paper in some manufactures (magazines, paper for printing, handwriting . . . ). The presence of mineral matter can be undesirable, in particular for the production of sanitary papers. In such cases their separation from the pulp is necessary.
The preparation of pulp using used paper is the global method, going from disintegration of the used paper until the different stages of decontamination, eventually the elimination of inks (de-inking) and the mineral matter (washing) and may include one or two whitening stages allowing the fibres to recover their initial whiteness. The obtained pulp will be used to feed the paper production apparatus.
The preparation of pulp for packing cardboards is much less demanding in terms of decontamination.
The classic method of pulp preparation from used paper always begins with the disintegration of the paper and the suspension of fibres, using a pulping apparatus (Stage A). The pulping apparatus is an equipment provided with a rotor (or turbine), which causes a sufficiently strong stirring to the used paper, blended with water, that the connections between the fibres (hydrogen bonds) are broken one after another. A paper pulp from used paper is thus obtained.
According to the quality of the final pulp to be obtained, the following stages may be followed:
Stage B: A coarse decontamination. The coarser components and in particular the plastic materials, are eliminated by passage through a grate.
Stage C: The elimination, by hydrocyclone, of the heavy coarse particles like coarse sands, bits of glass, and metallic particles such as clips.
Stage D: The elimination of the small plastics and other contaminants of intermediate size by the passage through a grate (or grate depuration) in two or three steps, consisting in passing the pulp through small holes (between 1 and 3 mm) and retaining the contaminants of size higher than the holes.
Stage E: The elimination of small contaminants, essentially of granular aspect (by opposition to plate contaminants) by a slot depuration (between 0.1 and 0.3 mm), working based in the same principle as the hole depuration. The holes are replaced by slots, which the fibres, taking into account their low diameter, are able to pass through.
Stage F: For the so-called  less than  less than white greater than  greater than  paper, the elimination of inks by one or several cells of flotation. Inks are separated with the help of small air bubbles eventually with the help of a soap or a surfactant.
Stage G: The elimination of the fine sands and coarse black points (small heavy contaminants) by batteries of several stages of hydrocyclones.
Stage H: In some cases, the elimination of small contaminants of density lower than 1 by a hydrocyclone.
Stage I: More particularly applicable to tissue paper, the elimination of the mineral matter by washing the pulp. The greater part of the water is evacuated, taking with it the major part of the loads.
Stage J: Thickening of the pulp, to facilitate its storage before reaching the paper apparatus or preparing the pulp by a hot dispersion or a refining.
Stage K: In some cases, dispersion of the residual contaminants, by one dispersant or grinder, in order to make these contaminants non visible by the eye. In other cases, modification of the pulp mechanical properties by one refiner.
In many cases, redilution and repeating one or more of the previous described stages is performed. It will then be called a second loop, or even a third loop, if after the second loop one or several stages are again repeated.
Stage L: Clarification of the filtrates by one microflotation device using dissolved air. The suspended matter is collected in flocks and then flotated at the surface with the help of air microbubbles and polymers (flocculants and coagulants).
Stage M: Thickening of the solid matters extracted in Stage L.
Stage N: Treatment of the residual waters by a depuration station.
Stage O: Whitening of the fibres for certain uses.
The presently used methods of pulp preparation are a more or less complete combination of the above-mentioned stages, each one performed by one different equipment. Between each stage, the pulp is generally pumped, causing a high energy consumption. Certain stages need the use of chemical products. It is frequent, particularly when the production of  less than  less than white greater than  greater than  paper is wanted, to impose restrictive optical criteria, and then the method of recycling used paper is not competitive in comparison with the utilisation of virgin cellulose.
The invention has the object of replacing itself, in its base version, to several equipments corresponding to the above-mentioned Stages H, J, L, M, or even C, D, E, F, G and I, whatever may be the type of paper to be produced, allowing also an important economy of energy and chemical products. The recycling of the used paper will then be more competitive, including when aimed to more exigent applications. The invention also allows a very lower land investment.